Switching converters are widely used to convert an input signal into an output signal through the ON and OFF switching of switches. A controller needing a DC power supply voltage, such as a control IC, is generally used to control the switches in the switching converter. When the switching converter is just started up, there needs some time (startup time) to establish the power supply voltage. During the startup time, the switching converter cannot work normally.
FIG. 1 illustrates a prior switching converter. A rectifier bridge is used to convert an AC input voltage VAC into a DC input voltage VDC at an output terminal. The switching converter comprising a switching circuit with a transformer T1, a switch M, a diode Dout and an output capacitor Cout converts the DC input voltage VDC into an output signal to drive a load. One terminal of a current source IS is coupled to the output terminal of the rectifier bridge. A capacitor C1 is coupled between the other terminal of the current source IS and the ground. A voltage across the capacitor C1 is used as a power supply voltage VCC of the control IC. A controller 101 is used to control the ON and OFF switching of the switch M. Internal components and circuit blocks in the controller 101 can be powered by the power supply voltage VCC.
FIG. 2 illustrates a waveform of the power supply voltage VCC during startup of the switching converter shown in FIG. 1. When the switching converter just starts up, the power supply voltage VCC has not been built up, the DC input voltage VDC is configured to charge the capacitor C1 through the current source IS, and the power supply voltage VCC is gradually increased. When the power supply voltage VCC is increased to reach a upper threshold voltage ISOFF at time t1, a hysteresis comparison signal LOCK from a hysteresis comparator COM is changed from logical low into logical high. The current source IS is turned OFF. At time t2, the power supply voltage VCC gradually decreases to a lower threshold voltage ISON since no operation of the switching circuit have begun. The hysteresis comparison signal LOCK is changed from logical high into logical low, and then the current source IS is turned ON and is configured to charge the capacitor C1 again. At time t3, the switching converter enters into normal operation, and instead of the current source IS, the auxiliary winding of the transformer T1 is used to provide power to the control IC through a diode D1.
In most applications, the DC input voltage VDC is not constant. If the DC input voltage VDC is too low, the switching converter won't work normally. In order to avoid this, a brown-in voltage setting of the DC input voltage VDC is important to prevent the switching converter from an abnormal brown-out operation. Generally, the controller 101 will be started when the DC input voltage VDC increases to higher than a brown-in threshold voltage VBI, and the controller 101 is disabled when the DC input voltage VDC decreases to less than a brown-out threshold voltage VBO, thus assuring that the switching converter is operated at a normal voltage range of the DC input voltage VDC.
However, the present inventor has recognized that if the startup of the controller 101 happens when the power supply voltage VCC decreases to approach the lower threshold voltage ISON, the internal components and/or circuit blocks in the controller 101 will suddenly consume a very high peak current, accordingly, the power supply voltage VCC cannot provide enough energy to make the switching converter enter into normal operation, thereby causing the startup failure which is undesirable.